A polishing apparatus for polishing a wafer often uses a spectroscopic monitoring system for the purpose of mainly monitoring the progress of polishing of a dielectric layer (transparent layer), or uses an eddy-current type monitoring system for the purpose of mainly monitoring the progress of polishing of a conductive layer (metal film). In the spectroscopic monitoring system, a light-emitting fiber and a light-receiving fiber are connected to a light source and a spectrometer, respectively, which are mounted in a polishing table. The front ends of these fibers function as a measuring section constituting a light-emitting section and a light-receiving section. The measuring section (i.e., the light-emitting section and the light-receiving section) is disposed at such a position as to scan a wafer surface each time the polishing table makes one revolution. In the case of the eddy-current type monitor, an exciting coil, a detection coil, etc. are provided as a measuring section.
In this type of monitoring system having the measuring section disposed in the polishing table, it is difficult to precisely control a measurement position on a wafer surface during polishing. Generally, a polishing head is structured such that a wafer is allowed to move slightly inside a retaining ring of the polishing head. With this structure, the wafer may be displaced radially relative to the center of the polishing head, or may gradually rotate over time relative to the polishing head. Accordingly, it is difficult to successively measure a predetermined position on the surface of the wafer. As a result, measurement data can vary greatly depending on which portion of a feature formed on the wafer surface is measured.
FIG. 18A is a graph showing a change in measured film thickness at an initial stage of polishing, and FIG. 18B is a graph showing a change in measured film thickness at an intermediate stage of polishing. The measured film thicknesses in the graphs are those in a measurement area at a distance of about 120 mm from the center of a 300-mm wafer. The wafer as a measurement object is a wafer having a plurality of raised portions on the surface, for example, a wafer having a cell array composed of a number of cells (memory cells) arranged in a matrix pattern.
A spectroscopic monitoring system having a xenon flash light source was used to measure a thickness of a film of the wafer. Measurement data, which was considered to represent film thicknesses of the raised portion, was extracted. Referring to FIG. 18A, the variation in the measured film thickness is small, and the measured film thickness decreases approximately linearly with the number of revolutions of the polishing table, i.e. with the elapse of polishing time. Referring to FIG. 18B, in contrast, while the measured film thickness decreases with the polishing time, the variation in the measured film thickness is large, thus making it difficult to control a film-thickness profile and detect a polishing end point based on each one of the measured film thicknesses.
FIG. 19A is a diagram showing a profile (i.e., a cross-sectional shape) of a raised portion at the initial stage of polishing, corresponding to FIG. 18A, and FIG. 19B is a diagram showing a profile (i.e., a cross-sectional shape) of the raised portion at the intermediate stage of polishing, corresponding to FIG. 18B. The profile shown in FIG. 19A is a profile of a raised portion 106 before polishing of the wafer and having a rectangular cross section. The profile shown in FIG. 19B is a profile of the raised portion 106 obtained when polishing of the wafer is once stopped after the wafer has been polished for a certain period of time. Trenches 110 are formed on both sides of the raised portion 106. The raised portion 106 is, for example, the above-described cell (memory cell).
As can be seen in FIGS. 19A and 19B, the cross-sectional shape of the raised portion, which is rectangular before polishing, becomes rounded at its corners with the progress of polishing. Therefore, the measured film thickness varies depending on the measurement position of the measuring section of the spectroscopic monitoring system. For example, in FIG. 19A, the film thickness in the central portion of the raised portion 106 is equal to the film thickness in the edge portion of the raised portion 106, while in FIG. 19B, the film thickness at a topmost portion 106a located at the center of the raised portion 106 differs from the film thickness in an edge portion 106b of the raised portion 106. As can be seen in FIG. 19B, the raised portion 106 has the maximum film thickness at its topmost portion 106a and the minimum film thickness in its edge portion 106b. Accordingly, the measured film thickness varies depending on the difference in the measurement position, thus making it impossible to accurately detect a polished state.